1. Field of the Invention
The present invention relates to a weight detecting apparatus for detecting weight mounting conditions of a crane equipped with weights such as a counterweight, and a crane having the same.
2. Description of the Related Art
The following descriptions will be made taking a crawler crane as an example.
There are some cranes in which a carbody weight for enhancing the stability of its machine body by increasing the weight of a base machine, is mounted in the vicinity of the pivot of a carbody in a lower traveling body (refer to Japanese Patent No. 3424616).
On the other hand, there are some cranes in which a crawler weight is mounted on a crawler portion of the lower traveling body, and a weight referred to as a dummy weight is mounted in a winch space in an upper rotating body. Hereinafter, these various weights may be generically called “weights” in a simple way.
In the crawler crane, there is provided an overload preventing apparatus (generally referred to as a “moment limiter”). By this apparatus, a crane capability is set based on a variety of factors, such as mounting conditions of a weight or weights (the presence/absence of weights (or a weight), the number of weights, the position and mass of each individual weight, etc.), the length and angle of a boom, and the weight of a hoist load, and concurrently, overload preventing operations are performed, such as monitoring of a load, warning, and automatic stop of machine, are performed to prevent the working load from exceeding the aforementioned crane capability.
Hitherto, as means for detecting weight mounting conditions constituting one of the factors determining the crane capability, there are known techniques as follows:    (i) a technique using a proximity switch or a limiting switch as means for detecting the presence/absence of weights (counterweights) [refer to Japanese Unexamined Patent Application Publication Nos. 10-203784 and 8-217382].    (ii) a technique for detecting the weight of weights (counterweights), or moment loads (refer to Japanese Patent No. 3351662).
According to the technique in the above (i), however, it is necessary to exactly maintain the relationship between the detector and a position of the weight. This makes the installation of the detector troublesome under constrain of its installation position, while a positional displacement might be caused by vibrations during working, thereby raising a possibility of an erroneous detection.
Also, the detector used here is a switch, and hence, even though it can make a simple detection such as detection of the presence/absence of weights (or a weight), it cannot detect other factors determining the crane capability, such as the number of weights, the position and weight of each individual weight, etc.
On the other hand, the technique in the above (ii) can detect the weights of weights and/or a moment load, but it involves problems in that an arrangement for detection becomes complicated, and the detection accuracy with respect to the increase/decrease in the total weight of the counterweights is difficult to secure (in particular, when the total weight of the counterweights is large, a small weight change is difficult to detect).
Furthermore, in either one of the cases (i) and (ii), when detecting mounting conditions of the weights mounted on the lower traveling body side, such as carbody weights or dummy weights, it is necessary to add a transmission device, such as a slip ring serving as means for sending detected information to an overload preventing apparatus provided on the upper rotating body side, so that the detection structure becomes complicated. This makes it very difficult to retrofit the detection structure.